Three-dimensional Vertical NOR Flash Thin-Film Transistor Strings

ABSTRACT

A memory structure, includes (a) active columns of polysilicon formed above a semiconductor substrate, each active column extending vertically from the substrate and including a first heavily doped region, a second heavily doped region, and one or more lightly doped regions each adjacent both the first and second heavily doped region, wherein the active columns are arranged in a two-dimensional array extending in second and third directions parallel to the planar surface of the semiconductor substrate; (b) charge-trapping material provided over one or more surfaces of each active column; and (c) conductors each extending lengthwise along the third direction. The active columns, the charge-trapping material and the conductors together form a plurality of thin film transistors, with each thin film transistor formed by one of the conductors, a portion of the lightly doped region of an active column, the charge-trapping material between the portion of the lightly doped region and the conductor, and the first and second heavily doped regions. The thin film transistors associated with each active column are organized into one or more vertical NOR strings.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention is a continuation application of U.S. patentapplication (“Parent Application I”), Ser. No. 16/107,732, entitled“Three-dimensional vertical NOR Flash Thin film Transistor Strings,”filed on Aug. 21, 2018, which is a continuation application of U.S.patent application (“Parent Application II”), Ser. No. 15/837,734,entitled “Three-dimensional vertical NOR Flash Thin film TransistorStrings,” filed on Dec. 11, 2017, now U.S. Pat. No. 10,096,364, which isa divisional application of U.S. patent application (“Parent ApplicationIII”), Ser. No. 15/343,332, entitled “Three-dimensional vertical NORFlash Thin film Transistor Strings,” filed on Nov. 4, 2016, now U.S.Pat. No. 9,842,651, which is related to and claims priority of (i) U.S.provisional patent application (“Provisional Application I”), Ser. No.62/260,137, entitled “Three-dimensional Vertical NOR Flash Thin-filmTransistor Strings,” filed on Nov. 25, 2015; (ii) U.S. non-provisionalpatent application (“Non-Provisional application I”), Ser. No.15/220,375, “Multi-Gate NOR Flash Thin-film Transistor Strings Arrangedin Stacked Horizontal Active Strips With Vertical Control Gates,” filedon Jul. 26, 2016; and (iii) U.S. provisional patent application(“Copending Provisional Application II”), Ser. No. 62/363,189, entitled“Capacitive Coupled Non-Volatile Thin-film Transistor Strings,” filedJul. 15, 2016; and (iv) U.S. non-provisional patent application(“Non-Provisional Patent Application II”), Ser. No. 15/248,420, entitled“Capacitive Coupled Non-Volatile Thin-film Transistor Strings inThree-Dimensional Array,” filed Aug. 26, 2016. The disclosures of ParentApplications I-III, Provisional Applications I-II, and Non-ProvisionalPatent Applications I-II are hereby incorporated by reference in theirentireties.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to high-density memory structures. Inparticular, the present invention relates to high-density memorystructures formed by interconnected thin film storage elements, such asthin film storage transistors formed in vertical strips with horizontalword lines.

2. Discussion of the Related Art

In this disclosure, memory circuit structures are described. Thesestructures may be fabricated on planar semiconductor substrates (e.g.,silicon wafers) using conventional fabrication processes. To facilitateclarity in this description, the term “vertical” refers to the directionperpendicular to the surface of a semiconductor substrate, and the term“horizontal” refers to any direction that is parallel to the surface ofthat semiconductor substrate.

A number of high-density non-volatile memory structures, such as“three-dimensional vertical NAND strings,” are known in the prior art.Many of these high-density memory structures are formed using thin filmstorage transistors formed out of deposited thin films (e.g.,polysilicon thin films), and organized as arrays of “memory strings.”One type of memory strings is referred to as NAND memory strings orsimply “NAND strings”. A NAND string consists of a number ofseries-connected thin film storage transistors (“TFTs”). Reading orprogramming the content of any of the series-connected TFTs requiresactivation of all series-connected TFTs in the string. Thin film NANDtransistors have lower conductivity than NAND transistors formed insingle crystal silicon, therefore the low read current that is requiredto be conducted through a long string of NAND TFTs results in arelatively slow read access (i.e. long latency).

Another type of high density memory structure is referred to as the NORmemory strings or “NOR strings.” A NOR string includes a number ofstorage transistors each of which is connected to a shared source regionand a shared drain region. Thus, the transistors in a NOR string areconnected in parallel, so that a read current in a NOR string isconducted over a much lesser resistance than the read current through aNAND string. To read or program a storage transistor in a NOR string,only that storage transistor needs to be activated (i.e., “on” orconducting), all other storage transistors in the NOR string may remaindormant (i.e., “off” or non-conducting). Consequently, a NOR stringallows much faster sensing of the activated storage transistor to beread. Conventional NOR transistors are programmed by a channelhot-electron injection technique, in which electrons are accelerated inthe channel region by a voltage difference between the source region andthe drain region and are injected into the charge-trapping layer betweenthe control gate and the channel region, when an appropriate voltage isapplied to the control gate. Channel hot-electron injection programmingrequires a relatively large electron current to flow through the channelregion, therefore limiting the number of transistors that can beprogrammed in parallel. Unlike transistors that are programmed byhot-electron injection, in transistors that are programmed byFowler-Nordheim tunneling or by direct tunneling, electrons are injectedfrom the channel region to the charge-trapping layer by a high electricfield that is applied between the control gate and the source and drainregions. Fowler-Nordheim tunneling and direct tunneling are orders ofmagnitude more efficient than channel hot-electron injection, allowingmassively parallel programming; however, such tunneling is moresusceptible to program-disturb conditions.

3-Dimensional NOR memory arrays are disclosed in U.S. Pat. No. 8,630,114to H. T Lue, entitled “Memory Architecture of 3D NOR Array,” filed onMar. 11, 2011 and issued on Jan. 14, 2014.

U.S. patent Application Publication US2016/0086970 A1 by Haibing Peng,entitled “Three-Dimensional Non-Volatile NOR-type Flash Memory,” filedon Sep. 21, 2015 and published on Mar. 24, 2016, discloses non-volatileNOR flash memory devices consisting of arrays of basic NOR memory groupsin which individual memory cells are stacked along a horizontaldirection parallel to the semiconductor substrate with source and drainelectrodes shared by all field effect transistors located at one or twoopposite sides of the conduction channel.

Three-dimensional vertical memory structures are disclosed, for example,in U.S. Pat. No. 8,878,278 to Alsmeier et al. (“Alsmeier”), entitled“Compact Three Dimensional Vertical NAND and Methods of Making Thereof,”filed on Jan. 30, 2013 and issued on Nov. 4, 2014. Alsmeier disclosesvarious types of high-density NAND memory structures, such as “terabitcell array transistor” (TCAT) NAND arrays (FIG. 1A), “pipe-shapedbit-cost scalable” (P-BiCS) flash memory (FIG. 1B) and a “vertical NAND”memory string structure. Likewise, U.S. Pat. No. 7,005,350 to Walker etal. (“Walker I”), entitled “Method for Fabricating Programmable MemoryArray Structures Incorporating Series—Connected Transistor Strings,”filed on Dec. 31, 2002 and issued on Feb. 28, 2006, also discloses anumber of three-dimensional high-density NAND memory structures.

U.S. Pat. No. 7,612,411 to Walker (“Walker II”), entitled “Dual-GateDevice and Method” filed on Aug. 3, 2005 and issued on Nov. 3, 2009,discloses a “dual gate” memory structure, in which a common activeregion serves independently controlled storage elements in two NANDstrings formed on opposite sides of the common active region.

3-Dimensional NOR memory arrays are disclosed in U.S. Pat. No. 8,630,114to H. T Lue, entitled “Memory Architecture of 3D NOR Array”, filed onMar. 11, 2011 and issued on Jan. 14, 2014.

A three-dimensional memory structure, including horizontal NAND stringsthat are controlled by vertical polysilicon gates, is disclosed in thearticle “Multi-layered Vertical gate NAND Flash Overcoming StackingLimit for Terabit Density Storage” (“Kim”), by W. Kim et al., publishedin the 2009 Symposium on VLSI Tech. Dig. of Technical Papers, pp188-189. Another three-dimensional memory structure, also includinghorizontal NAND strings with vertical polysilicon gates, is disclosed inthe article, “A Highly Scalable 8-Layer 3D Vertical-gate (VG) TFT NANDFlash Using Junction-Free Buried Channel BE-SONOS Device,” by H. T. Lueet al., published in the 2010 Symposium on VLSI: Tech. Dig. Of TechnicalPapers, pp. 131-132.

FIG. 1a shows three-dimensional vertical NAND strings 101 and 102 in theprior art. FIG. 1b shows basic circuit representation 140 of athree-dimensional vertical NAND string in the prior art. Specifically,vertical NAND string 101 and 102 of FIG. 1a and their circuitrepresentation 150 are each essentially a conventional horizontal NANDstring which—rather than each connecting 32 or more transistors inseries along the surface of a substrate—is rotated 90 degrees, so as tobe perpendicular to the substrate. Vertical NAND strings 101 and 102 areserially-connected thin film transistors (TFTs) in a stringconfiguration that rises like a skyscraper from the substrate, with eachTFT having a storage element and a control gate provided by one of theword line conductors in an adjacent stack of word line conductors. Asshown in FIG. 1b , in the simplest implementation of a vertical NANDstring, TFTs 15 and 16 are the first and last memory transistors of NANDstring 150, controlled by separate word lines WL0 and WL31,respectively. Bit line select transistor 11, activated by signal BLS,and ground select transistor 12, activated by signal SS, serve toconnect an addressed TFT in vertical NAND string 150 to correspondingglobal bit line GBL at terminal 14 and global source line (ground) GSL,at terminal 13, during read, program, program-inhibit and eraseoperations. Reading or programming the content of any one TFT, (e.g.,TFT 17) requires activation of all 32 TFTs in vertical NAND string 150,which exposes each TFT to read-disturb and program-disturb conditions.Such conditions limit the number of TFTs that can be provided in avertical NAND string to no more than 64 or 128 TFTs. Furthermore, thepolysilicon thin films upon which a vertical NAND string is formed havemuch lower channel mobility—and therefore higher resistivity—thanconventional NAND strings formed in a single-crystal silicon substrate,thereby resulting in a low read current relative to the read current ofa conventional NAND string.

U.S. Patent Application Publication 2011/0298013 (“Hwang”), entitled“Vertical Structure Semiconductor Memory Devices And Methods OFManufacturing The Same,” discloses three-dimensional vertical NANDstrings. In its FIG. 4D, Hwang shows a block of three dimensionalvertical NAND strings addressed by wrap-around stacked word lines 150(reproduced herein as FIG. 1c ).

U.S. Pat. No. 5,768,192 to Eitan, entitled “Memory Cell utilizingasymmetrical charge trapping” filed Jul. 23, 1996 and issued Jun. 16,1998 discloses NROM type memory transistor operation of the typeemployed in an embodiment of the current invention.

U.S. Pat. No. 8,026,521 to Zvi Or-Bach et al, entitled “SemiconductorDevice and Structure,” filed on Oct. 11, 2010 and issued on Sep. 27,2011 to Zvi-Or Bach et al discloses a first layer and a second layer oflayer-transferred mono-crystallized silicon in which the first andsecond layers include horizontally oriented transistors. In thatstructure, the second layer of horizontally oriented transistorsoverlays the first layer of horizontally oriented transistors, eachgroup of horizontally oriented transistors having side gates.

Transistors that have a conventional non-volatile memory transistorstructure but short retention times may be referred to as“quasi-volatile.” In this context, conventional non-volatile memorieshave data retention time exceeding tens of years. A planarquasi-volatile memory transistor on single crystal silicon substrate isdisclosed in the article “High-Endurance Ultra-Thin Tunnel Oxide inMonos Device Structure for Dynamic Memory Application”, by H. C. Wannand C. Hu, published in IEEE Electron Device letters, Vol. 16, No. 11,November 1995, pp 491-493. A quasi-volatile 3-D NOR array withquasi-volatile memory is disclosed in the U.S. Pat. No. 8,630,114 to H.T Lue, mentioned above.

SUMMARY

According to one embodiment of the present invention, a high densitymemory structure, referred to as a three-dimensional vertical NOR Flashmemory string (“multi-gate vertical NOR string,” or simply “vertical NORstring”). The vertical NOR string includes a number of thin filmtransistors (“TFTs”) connected in parallel, having a shared sourceregion and a shared drain region each extending generally in a verticaldirection. In addition, the vertical NOR string includes multiplehorizontal control gates each controlling a respective one of the TFTsin the vertical NOR string. As the TFTs in a vertical NOR string areconnected in parallel, a read current in a vertical NOR string isconducted over a much lesser resistance than the read current through aNAND string of a comparable number of TFTs. To read or program any oneof the TFTs in a vertical NOR string, only that TFT needs to beactivated, all other TFTs in the vertical NOR string can remainnon-conducting. Consequently, a vertical NOR string may include manymore TFTs (e.g., several hundreds or more), while allowing fastersensing and minimizing program-disturb or read-disturb conditions.

In one embodiment, the shared drain region of a vertical NOR string isconnected to a global bit line (“voltage V_(bl)”) and the shared sourceregion of the vertical NOR string is connected to a global source line(“voltage V_(ss)”). Alternatively, in a second embodiment, only theshared drain region is connected to a global bit line biased to a supplyvoltage, while the shared source region is pre-charged to a voltagedetermined by a quantity of charge in the shared source region. Toperform the pre-charge, one or more dedicated TFTs may be provided topre-charge the parasitic capacitance C of the shared source region.

According to one embodiment of the present invention, multi-gate NORflash thin film transistor string arrays (“multi-gate NOR stringarrays”) are organized as arrays of vertical NOR strings runningperpendicular to the surface of a silicon substrate. Each multi-gate NORstring array includes a number of vertical active columns arranged inrows, each row extending along a first horizontal direction, with eachactive column having two vertical heavily-doped polysilicon regions of afirst conductivity, which are separated by one or more verticalpolysilicon regions that are undoped or lightly doped to a secondconductivity. The heavily-doped regions each form a shared source ordrain region and, in conjunction with one or more stacks of horizontalconductors each extending orthogonally to the first horizontaldirection, the lightly-doped regions each form multiple channel regions.A charge-trapping material forms storage elements, covering at least thechannel regions of TFTs in the active column. The horizontal conductivelines in each stack are electrically isolated from each other and formcontrol gates over the storage elements and the channel regions of theactive column. In this manner, the multi-gate NOR string array forms athree-dimensional array of storage TFTs.

In one embodiment, support circuitry is formed in a semiconductorsubstrate to support multiple multi-gate NOR string arrays formed abovethe support circuitry and the semiconductor substrate. The supportcircuitry may include address encoders, address decoders, senseamplifiers, input/output drivers, shift registers, latches, referencecells, power supply lines, bias and reference voltage generators,inverters, NAND, NOR, Exclusive-Or and other logic gates, other memoryelements, sequencers and state machines, among others. The multi-gateNOR string arrays may be organized as multiple blocks of circuits, witheach blocks having multiple multi-gate NOR string arrays.

According to embodiments of the present invention, variations inthreshold voltages of TFTs within a vertical NOR string may becompensated by providing one or more electrically programmable referencevertical NOR strings in the same or another multi-gate vertical NORstring array. Background leakage currents inherent to a vertical NORstring can be substantially neutralized during a read operation bycomparing the results of the TFT being read to that of a TFT that isconcurrently read on a programmable reference vertical NOR string. Insome embodiments, each TFT of a vertical NOR string is shaped so as toamplify the capacitive coupling between each control gate and itscorresponding channel region thereby to enhance tunneling from thechannel regions into the charge-trapping material (i.e., the storageelement) during programming, and to reduce the charge injection from thecontrol gate to the charge-trapping material during erasing. Thisfavorable capacitive coupling is particularly useful for storing morethan one bit in each TFT of a vertical NOR string. In anotherembodiment, the charge-trapping material of each TFT may have itsstructure modified to provide a high write/erase cycle endurance, albeitat a lower retention time that requires refreshing of the stored data.However, as the refreshing required of a vertical NOR string array isexpected to be much less frequently than in a conventional dynamicrandom access memory (DRAM), the multi-gate NOR string arrays of thepresent invention may operate in some DRAM applications. Such use of thevertical NOR strings allows a substantially lower cost-per-bit figure ofmerit, as compared to conventional DRAMs, and a substantially lowerread-latency, as compared to conventional NAND string arrays.

In another embodiment the vertical NOR string can be programmed, erasedand read as NROM/Mirror-bit TFT string.

Organizing the TFTs as vertical NOR strings—rather than the prior artvertical NAND strings—results in (i) a reduced read-latency that canapproach that of a dynamic random access memory (DRAM) array, (ii)reduced sensitivities to read-disturb and program-disturb conditionsthat are associated with long NAND Flash strings, and (iii) reduced costper bit, as compared to a NAND Flash string.

The present invention is better understood upon consideration of thedetailed description below, in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows three-dimensional vertical NAND strings 101 and 102 in theprior art.

FIG. 1b shows basic circuit representation 140 of a three-dimensionalvertical NAND string in the prior art.

FIG. 1c shows a three-dimensional representation of a block ofthree-dimensional vertical NAND strings addressed by wrap-around stackedword lines 150.

FIG. 2 shows conceptualized memory structure 100, which illustrates a3-dimensional organization of memory cells; the memory cells areprovided in vertical NOR strings, with each vertical NOR string havingmemory cells each being controlled by one of a number of horizontal wordlines, according to one embodiment of the present invention.

FIG. 3a shows a basic circuit representation in a Z-Y plane of verticalNOR string 300 formed in an active column; vertical NOR string 300represents a three-dimensional arrangement of non-volatile storage TFTs,with each TFT sharing local source line (LSL) 355 and local bit line(LBL) 354, being accessed respectively by global bit line (GBL) 314 andglobal source line (GSL) 313 according to one embodiment of the currentinvention.

FIG. 3b shows a basic circuit representation in a Z-Y plane of verticalNOR string 305 formed in an active column; vertical NOR string 305represents a three-dimensional arrangement of non-volatile storage TFTs,including a dedicated pre-charge TFT 370 for setting a voltage(“V_(ss)”) on shared local source line 355, which has a parasiticcapacitance C, according to one embodiment of the present invention.

FIG. 3c shows a basic circuit representation of dynamic non-volatilestorage transistor 317 having one or more programmed threshold voltagesand connected to parasitic capacitor 360; capacitor 360 is pre-chargedto temporarily hold a virtual voltage V_(ss) on source terminal 355 soas to allow the threshold voltage of transistor 317 to be dynamicallydetected by the discharging of voltage V_(ss), when control gate 323 pis raised to a voltage that exceeds the threshold voltage.

FIG. 4a is a cross section in a Z-Y plane showing side-by-side activecolumns 431 and 432, each of which may form a vertical NOR string thathas a basic circuit representation illustrated in either FIG. 3a or FIG.3b , according to one embodiment of the present invention.

FIG. 4b is a cross section in the Z-X plane showing active columns 430R,430L, 431R and 431L, charge-trapping layers 432 and 434, and word lines423 p-L and 423 p-R, according to one embodiment of the presentinvention.

FIG. 4c shows a basic circuit representation in the Z-X plane ofvertical NOR string pairs 491 and 492, according to one embodiment ofthe present invention.

FIG. 5 is a cross section in the Z-Y plane showing connections ofvertical NOR string of active column 531 to global bit line 514-1(GBL1), global source line 507 (GSL1), and common body bias source 506(V_(bb)), according to one embodiment of the present invention.

FIG. 6a is a cross section in the X-Y plane showing, according to oneembodiment of the present invention, TFT 685 (T_(L)) of vertical NORstring 451 a and TFT 684 (T_(R)) of vertical NOR string 451 b invertical NOR string pair 491, as discussed in conjunction with FIG. 4c ;in FIG. 6a , global bit line 614-1 accesses alternate ones of local bitlines LBL-1, and predetermined curvature 675 of transistor channel 656Lamplifies the capacitive coupling between each control gate and thecorresponding channel during programming.

FIG. 6b is a cross section in the X-Y plane showing, according to oneembodiment of the current invention, TFT 685 (T_(L)) of vertical NORstring 451 a sharing an active region with TFT 684 (T_(R)) of verticalNOR string 451 b in vertical NOR string pair 491, as discussed inconjunction with FIG. 4c ; in FIG. 6b , global bit line 614-1 accessesalternate (odd) ones of local bit lines 654 (LBL-1), global bit line614-2 addresses alternate (even) ones of local bit lines 657-2 (LBL-2),local source lines LSL-1 and LSL-2 are pre-charged to provide virtualsupply voltage V_(ss).

FIG. 6c is a cross section in the X-Y plane showing, in accordance withone embodiment of the current invention, dedicated word line stacks 623p, each having word lines each surrounding (“wrapping around”) a TFT ofa vertical NOR string, and local vertical pillar bit line 654 (extendingalong the Z direction) and local vertical pillar source line 655(extending along the Z direction), which are accessed by globalhorizontal bit line 614 and global horizontal source line 615,respectively; in FIG. 6c , adjacent word line stacks 623 p are isolatedfrom each other by air gap 610 or another dielectric isolation.

FIG. 6d is a cross section in the X-Y plan showing, according to theembodiment of the present invention, staggered close-packing of verticalNOR strings, similar to those shown in FIG. 6c , sharing word-linestacks 623 p and with pre-charged parasitic capacitors 660 eachproviding a pre-charged virtual V_(ss) supply voltage.

FIGS. 7a, 7b, 7c and 7d are cross sections of intermediate structuresformed in a fabrication process for a multi-gate NOR string array, inaccordance with one embodiment of the present invention.

FIG. 8a is a schematic representation of a read operation forembodiments where the local source line (LSL) of a vertical NOR stringis hard-wired; in FIG. 8a , “WLs” represents the voltage on the selectedword line, and all non-select word lines (“WL_(Ns)”) in the vertical NORstring are set at 0V during the read operation.

FIG. 8b is a schematic representation of a read operation forembodiments where the local source line is floating at pre-chargevirtual voltage V_(ss); in FIG. 8b , “WL_(CHG)” represents the gatevoltage on the pre-charge transistor (e.g., pre-charge transistor 317 or370 in FIG. 3c ).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows conceptualized memory structure 100, which illustrates a3-dimensional organization of memory cells (or storage elements)provided in vertical NOR strings. In conceptualized memory structure100, each vertical NOR string includes memory cells that are eachcontrolled by a corresponding horizontal word line, according to oneembodiment of the present invention. In conceptualized memory structure100, each memory cell is formed in deposited thin films provided“vertically”, i.e., along a direction perpendicular to the surface ofsubstrate layer 101. Substrate layer 101 may be, for example, aconventional silicon wafer used for fabricating integrated circuits,familiar to those of ordinary skill in the art. In this detaileddescription, a Cartesian coordinate system (such as indicated in FIG. 2)is adopted solely for the purpose of facilitating discussion. Under thiscoordinate system, the surface of substrate layer 101 is considered aplane which is parallel to the X-Y plane. Thus, as used in thisdescription, the term “horizontal” refers to any direction parallel tothe X-Y plane, while “vertical” refers to the Z-direction.

In FIG. 2, each vertical column in the Z direction represents storageelements or TFTs in a vertical NOR string (e.g., vertical NOR string121). The vertical NOR strings are arranged in a regular manner in rowseach extending along the X direction. (Of course, the same arrangementmay also be seen alternatively as an arrangement of rows each extendingalong the Y directions). The storage elements of a vertical NOR stringshare a vertical local source line and a vertical local bit line (notshown). A stack of horizontal word lines (e.g., WL 123) run along the Ydirection, with each word line serving as control gates forcorresponding TFTs of vertical NOR strings located adjacent the wordline along the Y direction. Global source lines (e.g., GSL 122) andglobal bit lines (e.g., GBL 124) are provided along the X directiongenerally running either below the bottom of or on top of conceptualizedmemory structure 100. Alternatively, signal lines GSL 122 and GBL 124can both be routed below or both be routed on top of conceptualizedmemory structure 100, each of these signal lines may be selectivelyconnected by access transistors (not shown) to the local source linesand local bit lines of individual vertical NOR strings. Unlike avertical NAND string of the prior art, in a vertical NOR string of thepresent invention, writing or reading any one of its storage elementsdoes not involve activation of any other storage element in the verticalNOR string. As shown in FIG. 2, solely for illustrative purpose,conceptualized memory block 100 is a multi-gate vertical NOR stringarray consisting of a 4×5 arrangement of vertical NOR strings, with eachNOR string typically having 32 or more storage elements and accessselection transistors. As a conceptualized structure, memory block 100is merely an abstraction of certain salient characteristics of a memorystructure of the present invention. Although shown in FIG. 2 as a 4×5arrangement of vertical NOR strings, with each vertical NOR stringshaving a number of storage elements, a memory structure of the presentinvention may have any number of vertical NOR strings in each row alongeither of the X and Y directions, and any number of storage elements ineach vertical NOR string. For example, there may be thousands ofvertical NOR strings arrayed in rows along both the X and Y directions,with each NOR string having, for example, 2, 4, 8, 16, 32, 64, 128, ormore storage elements.

The number of storage elements in each vertical NOR string of FIG. 2(e.g., vertical NOR string 121) corresponds to the number of word lines(e.g., WL 123) providing control gates to the vertical NOR string. Theword lines are formed as narrow, long metallic strips each extendingalong the Y direction. The word lines are stacked one on top of eachother, and electrically isolated from each other by dielectricinsulation layers there-between. The number of word lines in each stackmay be any number, but preferably an integer power of 2 (i.e., 2^(n),where n is an integer). The selection of a power of 2 for the number ofword lines follows a customary practice in conventional memory design.It is customary to access each addressable unit of memory by decoding abinary address. This custom is a matter of preference and need not befollowed. For example, within the scope of the present invention,conceptualized memory structure 100 may have M vertical NOR stringsalong each row in the X and Y directions, with M being a number that isnot necessarily 2^(n), for any integer n. In some embodiments to bedescribed below, two vertical NOR strings may share a vertical localsource line and a vertical local bit line, but their respective storageelements are controlled by two separate word line stacks. Thiseffectively doubles the storage density of the vertical NOR string.

As conceptualized memory structure 100 of FIG. 2 is provided merely toillustrate an organization of memory cells, it is not drawn to specificscale in any of the X, Y, Z directions.

FIG. 3a shows a basic circuit representation in a Z-Y plane of verticalNOR string 300 formed in an active column; vertical NOR string 300represents a three-dimensional arrangement of non-volatile storage TFTs,with each TFT sharing local source line 355 and local bit line 354,according to one embodiment of the current invention. In this detaileddescription, the term “active region,” “active column” or “active strip”refers to a region, column or strip of one or more semiconductormaterials on which an active device (e.g., a transistor or a diode) maybe formed. As shown in FIG. 3a , vertical NOR string 300 runs along theZ direction, with TFTs 316 and 317 connected in parallel betweenvertical local source line 355 and vertical local drain or bit line 354.Bit line 354 and source line 355 are spaced apart, with the regionthere-between (i.e., body region 356) providing channel regions for theTFTs in the vertical NOR string. Storage elements are formed at theintersections between channel region 356 and each horizontal word line323 p, where p is the index of the word line in the word line stack; inthis example, p may take any value between 0 and 31. The word linesextend along the Y direction. Local bit line 354 is connected throughbit line access select transistor 311 to horizontal global bit line(GBL) 314, which runs along the X direction and connects local bit line354 to access bit line supply voltage V_(bl). Local source line 355 isconnected through horizontal global source line (GSL) 313 to sourcesupply voltage V_(ss). An optional source-select transistor (not shownin FIG. 3a ) can be provided to connect between local source line 355and GSL 313. The optional source-select transistor may be controlled bysource decoding circuitry which can be implemented either in thesubstrate (e.g., semiconductor substrate 101 of FIG. 2) or above thesubstrate and below memory structure 100, as is known to a personskilled in the art. Body region 356 of the active column may beconnected at terminal 331 to substrate bias voltage V_(bb). Substratebias voltage V_(bb) may be used, for example during an erase operation.The V_(bb) supply voltage can be applied to an entire multi-gatevertical NOR string array, or decoded so as to be applied selectively toone or more rows of vertical NOR strings. Lines connecting the V_(bb)supply voltage to body region 356 run preferably along the direction ofthe word lines.

FIG. 3b shows a basic circuit representation in a Z-Y plane of verticalNOR string 305 formed in an active column; vertical NOR string 305represents a three-dimensional arrangement of non-volatile storage TFTs,including (optionally) dedicated pre-charge TFT 370 for momentarilysetting a voltage (“V_(ss)”) on shared local source line 355, which hasa parasitic capacitance C, represented by capacitor 360, according toone embodiment of the present invention. Unlike vertical NOR string 300of FIG. 3a , vertical NOR string 305 does not implement GSL 313,replacing it with pre-charge transistor 370 which pre-charges parasiticcapacitor 360, which temporarily holds a voltage of V_(ss) volts. Underthis pre-charging scheme, global source lines (e.g., global source lines313 of FIG. 3a ) and its decoding circuitry are rendered unnecessary,thereby simplifying both the manufacturing process as well as circuitlayout, and providing a very tight footprint for each vertical NORstring. FIG. 3c highlights the structure of non-volatile storage TFT317, which can also be used, in addition to its normal storage function,to perform the pre-charge function of dedicated pre-charge transistor370. A dynamic read operation for TFT 317 is described below inconjunction with sensing the correct one of several threshold voltagesthat is programmed into storage element 334 of TFT 317.

FIG. 4a is a cross section in a Z-Y plane showing side-by-side activecolumns 431 and 432, each of which may form a vertical NOR string thathas a basic circuit representation illustrated in either FIG. 3a or FIG.3b , according to one embodiment of the present invention. As shown inFIG. 4a , active columns 431 and 432 each include vertical N+ dopedlocal source region 455 and vertical N+ doped local drain or bit lineregion 454, separated by lightly P− doped or undoped channel region 456.P− doped channel region 456, N+ doped local source region 455 and N+doped local drain or bit line region 454 may be biased to body biasvoltage V_(bb), source supply voltage V_(ss), and bit line voltageV_(bl), respectively. In some embodiments of the current invention, useof body bias voltage V_(bb) is optional, such as when the active stripis sufficiently thin (e.g., 10 nanometers or less). For a sufficientlythin active strip, the active region is readily fully depleted underappropriate voltage on the control gate, such that voltage V_(bb) maynot provide a solid supply voltage to the channel regions of the TFTsalong the vertical NOR string. Isolation region 436, which electricallyinsulates active columns 431 and 432, may be either a dielectricinsulator or an air-gap. A vertical stack of word lines 423 p,respectively labeled WL0-WL31 (and optionally WL_(CHG)), providescontrol gates to the TFTs in the vertical NOR strings formed in activecolumns 431 and 432. Word line stack 423 p is typically formed as longnarrow metallic conductors (e.g., tungsten, a silicide or silicide) thatextend along the Y direction, electrically isolated from each other bydielectric layers 426, each typically formed out of silicon oxide (e.g.,SiO₂) or an air gap. A non-volatile storage element may be formed at theintersection of each word line 423 p and each P− doped channel region456 by providing a charge-trapping material (not shown) between wordline 423 p and P− doped channel region 456. For example, FIG. 4aindicates by dashed boxes 416 the locations where nonvolatile storageelements (or storage transistors) T₀ to T₃₁ may be formed. Dashed box470 indicates where a dedicated pre-charge transistor may be formed,which, when momentarily switched on, allows charge to be transferredfrom common local bit line region 454 to common local source line region455 when all transistors T₀ to T₃₁ are in their off state.

FIG. 4b is a cross section in the Z-X plane showing active columns 430R,430L, 431R and 431L, charge-trapping layers 432 and 434, and word linestacks 423 p-L and 423 p-R, according to one embodiment of the presentinvention. Similar to FIG. 4a , each of vertical word line stacks 423p-L and 423 p-R in FIG. 4b denotes a stack of long narrow conductors,where p is an index labeling the word lines in stack (e.g., word linesWL0 to WL31). As shown in FIG. 4b , each word line serves as controlgates for the nonvolatile TFTs in the vertical NOR strings formed onadjacent active columns 430-L and 431-R on opposite sides of the wordline (within region 490). For example, in FIG. 4b , word line WL31 inword line stack 423 p-R serves as control gates for both transistor 416Lon active column 430L and transistor 416R on active column 431R.Adjacent word line stacks (e.g., word lines stacks 423 p-L and 423 p-R)are separated by a distance 495, which is the width of a trench formedby etching through successive word line layers, as described below.Active columns 430R and 430L, and their respective charge-trappinglayers 432 and 434, are subsequently formed inside the trench etchedthrough the word line layers. Charge-trapping layer 434 is providedinterposed between word line stack 423 p-R and vertical active columns431R and 430L. As elaborated below, during programming of transistor416R, charge injected into charge-trapping layer 434 is trapped in theportion of charge-trapping layer 434 within dash box 480. The trappedcharge alters the threshold voltage of TFT 416R, which may be detectedby measuring a read current flowing between local source region 455 andlocal drain region 454 on active column 431R (these regions are shown,e.g., FIG. 4a in the orthogonal cross section of the active column). Insome embodiments, pre-charge word line 478 (i.e., WL_(CHG)) is providedas control gate of pre-charge TFT 470 that is used to charge parasiticcapacitance C of local source line 455 (see, capacitor 360 of FIG. 3band local source line 455 of FIG. 4a ) to a ground or source supplyvoltage V_(ss). For expediency, charge-trapping layer 434 also providesa storage element in pre-charge transistor 470, which however is notitself used as a memory transistor. Pre-charging may alternatively beperformed using any of memory transistors T₀ to T₃₁ formed on activecolumn 431R. One or more of these memory transistors, in addition totheir storage function, can perform the function of the pre-chargetransistor. To perform the pre-charge, the voltage on the word line orcontrol gate is temporarily raised to a few volts above its highestprogrammable threshold voltage, thereby allowing voltage V_(ss) appliedto local bit line 454 to be transferred to local source line 455 (FIG.4a ). Having memory transistors T₀ to T₃₁ perform the pre-chargefunction eliminates the need for separate dedicated pre-charge TFT 470.Care must be taken, however, to avoid unduly disturbing the thresholdvoltage of such memory TFT when it is performing its pre-chargingfunction.

Although active columns 430R and 430L are shown in FIG. 4b as twoseparate active columns separated by an air-gap or dielectric insulation433, the adjacent vertical N+ local source lines may be implemented by asingle shared vertical local source line. Likewise, the vertical N+local drain or bit lines may be implemented by a single shared verticallocal bit line. Such a configuration provides “vertical NOR stringpair”. In that configuration, active columns 430L and 430R may be seenas two branches (hence the “pair”) in one active column. The verticalNOR string pair provides double-density storage through charge-trappinglayers 432 and 434 interposed between active columns 430R and 430L andword lines stacks 423 p-L and 423 p-R on opposite sides. In fact, activecolumns 430R and 430L may be merged into one active string byeliminating the air gap or dielectric insulation 433, yet still achievethe pair of NOR TFT strings implemented at the two opposite faces of thesingle active column. Such a configuration achieves the samedouble-density storage, as the TFTs formed in the opposite faces of theactive columns are controlled by separate word line stacks and areformed out of separate charge-trapping layers 434 and 432. Maintainingseparate thin active columns 430R and 430L (i.e., instead of mergingthem into one active column) is advantageous because TFTs on each activecolumn are thinner than the merged column, and can therefore morereadily be fully depleted under appropriate control gate voltageconditions, thereby to substantially cut down source-drain subthresholdleakage current between vertical source regions 455 and vertical drainregions 454 of the active columns (FIG. 4a ). Having ultra-thin (andtherefore highly resistive) active columns is possible for even verylong vertical NOR strings (e.g., 128 TFTs or longer) because the TFTs ina vertical NOR string are connected in parallel and because only one ofthe many TFTs is switched on at any one time, in contrast with the highresistance of a NAND TFT string where TFTs in the string are connectedin series and must therefore all be switched on to sense any one of TFTsin the string. For example, in a 32-TFT vertical NOR string, to be ableto read transistor T₃₀ (FIG. 4a ), the channel length of channel region456 may span just 20 nanometers, as compared to the correspondingchannel length of a NAND string, which may be 32 times longer, or 640nanometers.

FIG. 4c shows a basic circuit representation in the Z-X plane ofvertical NOR string pairs 491 and 492, according to one embodiment ofthe present invention. As shown in FIG. 4c , vertical NOR strings 451 band 452 a share a common word line stack 423 p-R, in the manner shownfor the vertical NOR strings of active strips 430L and 431R of FIG. 4b .For their respective commonly-connected local bit lines, vertical NORstring pairs 491 and 492 are served by global bit line 414-1 (GBL1)through access select transistor 411 and global bit line 414-2 (GBL2)through access select transistor 414, respectively. For their respectivecommonly-connected local source lines, vertical NOR string pairs 491 and492 are served by global source line 413-1 (GSL1) and global source line413-2 (GSL2), respectively (source line select access transistors can besimilarly provided, and are not shown in FIG. 4c ). As shown in FIG. 4c, vertical NOR string pair 491 includes vertical NOR strings 451 a and451 b that share local source line 455, local bit line 454, and optionalbody connection 456. Thus, vertical NOR string pair 491 represent thevertical NOR strings formed on active columns 430R and 430L of FIG. 4b .Word line stacks 423 p-L and 423 p-R (where, in this example, 31≥p≥0)provide control gates for vertical NOR string 451 a and vertical NORstring 451 b, respectively. The word lines to control gates in the stackare decoded by decoding circuitry formed in the substrate to ensure thatappropriate voltages are applied to the addressed TFT (i.e., theactivated word line) and to the unaddressed TFTs (i.e., all othernon-activated word lines in the string). FIG. 4c illustrates how storagetransistors 416L and 416R on active columns 430L and 431R of FIG. 4b areserved by the same word line stack 423 p-R. Thus, vertical NOR string451 b of vertical NOR string pair 491 and vertical NOR string 452 a ofvertical string pair 492 correspond to the adjacent vertical NOR stringsformed on active columns 430L and 431R of FIG. 4b . Storage transistorsof vertical NOR string 451 a (e.g., storage transistor 415R) are servedby word line stack 423 p-L.

In another embodiment, the hard-wired global source lines 413-1, 413-2of FIG. 4c are eliminated, to be substituted for by parasiticcapacitance C (e.g., capacitor 460 of FIGS. 4c and 360 of FIG. 3c )between shared N+ local source line 455—which is common to both verticalNOR strings 451 a and 451 b—and its numerous associated word lines 423p-L and 423 p-R. In a vertical stack of 32 TFTs, each of the 32 wordlines contribute their parasitic capacitance to provide total parasiticcapacitance C, such that it is sufficiently large to temporarily holdthe voltage supplied by pre-charge TFT 470 to provide a virtual sourcevoltage V_(ss) during the relatively short duration of read orprogramming operations. In this embodiment, the virtual source voltagetemporarily held on capacitor C is provided to local source line 455from global bit line GBL1 through access transistor 411 and pre-chargetransistor 470. Alternatively, dedicated pre-charge transistor 470 canbe eliminated, if one or more of the memory TFTs in the vertical NORsting are used, in addition to their storage function, to pre-chargelocal source line 455, by bringing its word line voltage momentarilyhigher than its highest programmed voltage. Using a storage TFT for thispurpose, care must be taken, however, to avoid over-programming thestorage TFT. Using the virtual V_(ss) voltage provides the significantadvantage of eliminating hard-wired global source lines (e.g., GLS1,GLS2) and their associated decoding circuitry and access transistors,thereby materially simplifying the process flow and design challengesand resulting in a significant more compact vertical NOR string.

FIG. 5 is a cross section in the Z-Y plane showing connections ofvertical NOR string of active column 531 to global bit line 514-1(GBL1), global source line 507 (GSL1), and common body bias source 506(V_(bb)), according to one embodiment of the present invention. As shownin FIG. 5, bit-line access select transistor 511 connects GBL1 withlocal bit line 554, and buried contact 556 optionally connects a P− bodyregion on the active strip to body bias source 506 (V_(bb)) in thesubstrate. Bit-line access select transistor 511 is formed in FIG. 5above active column 531. However, alternatively, bit-line access selecttransistor 511 may be formed at the bottom of active column 531 or insubstrate 505 (not shown in FIG. 5). In FIG. 5, bit-line access selecttransistor 511 can for example be formed in an isolated island of anN+/P−/N+ doped polysilicon stack together with access select word line585. When a sufficiently large voltage is applied to select word line585, the P− channel is inverted, thereby connecting local bit line 554to GBL1. Word line 585 runs along the same direction (i.e., the Ydirection) as the word lines 523 p which serve as control gates to theTFTs of the vertical NOR string. Word line 585 may be formed separatelyfrom word lines 523 p. In one embodiment, GBL1 runs horizontally alongthe X direction (i.e., perpendicular to the directions of the wordlines), and bit-line access select transistor 511 provides access tolocal bit line 554, which is the local bit line of merely one of manyvertical NOR strings that are served by GBL1. To increase read andprogram operation efficiency, in a multi-gate NOR string array,thousands of global bit lines may be used to access in parallel thelocal bit lines of thousands of vertical NOR strings that are accessedby word line 585. In FIG. 5, local source line 555 is connected throughcontact 557 to global source line 513-1 (GSL1), which may be decoded,for example by decoding circuitry in substrate 505. Alternatively, asdescribed already, the global source line may be eliminated by providinga virtual source voltage V_(ss) on local bit line 555 and temporarilypre-charging the parasitic capacitor 560 (i.e., parasitic capacitance C)of local source line 555 through TFT 570.

Support circuitry formed in substrate 505 may include address encoders,address decoders, sense amplifiers, input/output drivers, shiftregisters, latches, reference cells, power supply lines, bias andreference voltage generators, inverters, NAND, NOR, Exclusive-Or andother logic gates, other memory elements, sequencers and state machines,among others. The multi-gate NOR string arrays may be organized asmultiple blocks of circuits, with each block having multiple multi-gateNOR string arrays.

FIG. 6a is a cross section in the X-Y plane, showing TFT 685 (T_(L)) ofvertical NOR string 451 a and TFT 684 (T_(R)) of vertical NOR string 451b in vertical NOR string pair 491, as discussed above in conjunctionwith FIG. 4c . As shown in FIG. 6, TFTs 684 and 685 share N+ localsource region 655 and N+ local drain or bit line region 654, bothregions extending in long narrow pillars along the Z direction. (N+local source region 655 corresponds to local source line 455 of FIG. 4a, N+ local drain region 654 corresponds to local bit line 454 of FIG. 4a). In this embodiment, P− doped channel regions 656L and 656R form apair of active strings between local source pillar 655 and local drainpillar 654 and extend along the Z direction, isolated from each other byisolation region 640. Charge-trapping layer 634 is formed between wordlines 623 p-L (WL31-0) and 623 p-R (WL31-1) and the outside of channelregions 656L and 656R respectively. Charge trapping layer 634 may be atransistor gate dielectric material consisting of, for example, a thinfilm of tunnel dielectric (e.g., silicon dioxide), followed by a thinlayer of charge trapping material such as silicon nitride or conductivenanodots embedded in a non-conducting dielectric material, or isolatedfloating gates, and is capped by a layer of blocking dielectric such asONO (Oxide-Nitride-Oxide) or a high dielectric constant film such asaluminum oxide or hafnium oxide or some combination of such dielectrics.Source-drain conduction is controlled by word lines 623 p-L and 623 p-R,respectively, forming control gates on the outside of charge-trappinglayer 634. When programming or reading TFT 684 (T_(R)), TFT 685 (T_(L))is turned off by maintaining an appropriate inhibit voltage at word line623 p-L. Similarly, when programming or reading TFT 685 (T_(L)), TFT 684(T_(R)) is turned off by maintaining an appropriate inhibit voltage atword line 623 p-R.

In the embodiment shown in FIG. 6a , word lines 623 p-L and 623 p-R arecontoured to enhance tunneling efficiency into the TFTs 684 and 685during programming, while reducing reverse-tunneling efficiency duringerasing. Specifically, as is known to a person skilled in the art,curvature 675 of channel region 656R amplifies the electric field at theinterface between the active channel polysilicon and the tunnelingdielectric during programming, while reducing the electric field at theinterface between the word line and the blocking dielectric duringerasing. This feature is particularly helpful when storing more than onebit per TFT transistor in a multi-level cell (MLC) configuration. Usingthis technique, 2, 3, or 4 bits or more may be stored in each TFT. Infact, TFTs 684 and 685 may be used as analog storage TFTs with acontinuum of stored states. Following a programming sequence (to bediscussed below), electrons are trapped in charge-trapping layer 634, asindicated schematically by dashed lines 680. In FIG. 6a , global bitlines 614-1 and 614-2 run perpendicularly to word lines 623 p-R and 623p-L, and are provided either above or underneath the vertical NORstrings, corresponding to bit lines 414-1 and 414-2 respectively of FIG.4c . As discussed above in conjunction with FIG. 2, the word lines mayspan the entire length of memory block 100 along the X direction, whilethe global bit lines span the width of memory block 100 along the Ydirection. Of importance, in FIG. 6a , word line 623 p-R is shared byTFTs 684 and 683 of two vertical NOR strings on opposite sides of wordline 623 p-R. Accordingly, to allow TFTs 684 and 683 to be read orprogrammed independently, global bit line 614-1 (GBL1) contacts localdrain or bit line region 657-1 (“odd addresses”), while global bit line614-2 (GBL2) contacts local drain or bit line region 657-2 (“evenaddresses”). To achieve this effect, contacts along global bit lines614-1 and 614-2 are staggered, with each global bit line contactingevery other one of the vertical NOR string pair along the X-directionrow.

In like manner, global source lines (not shown in FIG. 6a ), which maybe located either at the bottom or above the multi-gate NOR stringarray, may run parallel to the global bit lines and may contact thelocal source lines of vertical NOR string pairs according to even or oddaddresses. Alternatively, where pre-charging of parasitic capacitance C(i.e., capacitor 660) temporarily to virtual source voltage V_(ss) isused, the global source lines need not be provided, thereby simplifyingthe decoding scheme as well as the process complexity.

FIG. 6a shows only one of several possible embodiments by which verticalNOR string pairs may be provided with stacked word lines. For example,curvature 675 in channel region 656R can be further accentuated.Conversely such curvature can be altogether eliminated (i.e.straightened out) as shown in the embodiment of FIG. 6b . In theembodiment of FIG. 6b isolation spacing 640 of FIG. 6a may be reduced oraltogether eliminated by merging channel regions 656L and 656R into asingle region 656(L+R), achieving greater area efficiency withoutsacrificing the dual-channel configuration: for example TFTs 685 (T_(L))and 684 (T_(R)) reside on opposite faces of the same active strip. Inthe embodiments of FIGS. 6a, 6b , vertical NOR strings sharing a wordline may be laid out in a staggered pattern relative to each other (notshown), such that they may be brought closer to each other, so as toreduce the effective footprint of each vertical NOR string. AlthoughFIGS. 6a and 6b show direct connection via a contact between global bitline 614-1 and N+ doped local drain bit line pillar 654 (LBL-1), suchconnection can also be accomplished using a bit-line access selectiontransistor (e.g., bit line access select transistor 511 of FIG. 5, notshown in already crowded FIGS. 6a and 6b ).

In the embodiments of FIGS. 6a and 6b , dielectric isolation between N+doped local drain region 654 and its adjacent local N+ doped sourceregion 658 (corresponding to isolation region 436 of FIG. 4a ) can beestablished by, for example, defining the separation 676 between wordlines 623 p-R and 623 p-L to be less than the thicknesses of twoback-to-back charge-trapping layers, so that the charge-trapping layersare merged together during their deposition. The resulting merging ofthe deposited charge-trapping layers creates the desired dielectricisolation. Alternatively, isolation between adjacent active strings canbe achieved by using a high aspect-ratio etch of N+ polysilicon tocreate gap 676 (air gap or dielectric filled) isolating N+ pillar 658 ofone string from N+ pillar 654 of the adjacent string (i.e., creating gap436 shown in FIG. 4a ).

Contrasting between the prior art vertical NAND strings and the verticalNOR strings of the current invention, although both types of devicesemploy thin film transistors with similar word line stacks as controlgates, their transistor orientations are different: in the prior artNAND string, each vertical active strip may have 32, 48 or more TFTsconnected in series. In contrast, each active column forming thevertical NOR strings of the present invention the vertical column mayhave one or two sets of 32, 48 or more TFTs connected in parallel. Inthe prior art NAND strings, the word lines in some embodiments typicallywrap around the active strip. In some embodiments of the vertical NORstring of the present invention separate designated left and right wordlines are employed for each active strip, thereby to achieve a doubling(i.e. a pair) storage density for each global bit line, as illustratedin FIGS. 4c, 6a and 6b . The vertical NOR strings of the presentinvention do not suffer from program-disturb or read-disturbdegradation, nor do they suffer from the slow latency of the prior artNAND strings. Thus, a much larger number of TFTs may be provided in avertical NOR string than in a vertical NAND strings. Vertical NORstrings, however, may be more susceptible to subthreshold or otherleakage between the long vertical source and drain diffusions (e.g.,local source region 455 and local drain region 454, respectively,illustrated in FIG. 4a ).

Two additional embodiments of the vertical NOR string of this inventionare shown in FIG. 6c and FIG. 6d . In these embodiments, all word linesin each word-line stack wrap around the vertical active strip.

In FIG. 6c , a vertical NOR string is formed inside the voids that areformed by etching through a stack of metal word lines and the dielectricisolation layers between the word lines. The manufacturing process flowis similar to that of the prior art vertical NAND strings, except thatthe transistors in a vertical NOR string are provided parallel to eachother, rather than serially in a vertical NAND string. Formation oftransistors in a vertical NOR string is facilitated by the N+ dopedvertical pillars extending to the entire depth of the void, providingshared local source line 655 (LSL) and shared local bit line (drain) 654(LBL) for all the TFTs along the vertical NOR string, with undoped orlightly doped channel region 656 adjacent to both. Charge storageelement 634 is positioned between channel 656 and word line stack 623 p,thus forming a stack of 2, 4, 8, . . . 32, 64 or more TFTs (e.g., device685 (T₁₀)) along the vertical active strip. In the embodiment of FIG. 6c, the word line stacks run in the Y direction, with individualhorizontal strips 623 p (WL31-0), 623 p (WL31-1) being separated fromeach other by air gap or dielectric isolation 610. Global bit lines 614(GBL) and global source lines 615 (GSL) run horizontally in rows alongthe X direction, perpendicular to the word lines. Each global bit line614 accesses local bit line pillars 654 (LBL) along the row of verticalstrips through access select transistors (511 in FIG. 5, not shown here)that can be positioned either below the memory array or above it.Similarly, each global source line 615 accesses the local source linepillars along the row. While the structures shown in FIGS. 6a and 6b areable to fit a pair of vertical NOR strings in roughly the same areataken up by a single vertical NOR string in the embodiment of FIG. 6c ,each TFT in each vertical NOR string shown in FIG. 6c has two parallelconduction channels (i.e., channel regions 656 a and 656 b), andtherefore may store more charge and increase or double the read current,thereby enabling storing more bits in each TFT.

FIG. 6d shows a more compact vertical NOR string with wrap-around wordlines, according to one embodiment of the present invention. As shown inFIG. 6d , vertical NOR strings are staggered as to be closer together,so that word line stack 623 p (WL31-0) can be shared by more verticalNOR strings. The staggered configuration is enabled by the use ofparasitic capacitance C (i.e., capacitors 660) of local source linepillar 655 (LSL). By pre-charging capacitors 660 to temporarily holdvirtual voltage V_(ss) during read and program operations, as describedbelow, the need for hard-wired global source lines (e.g., GSL 615 inFIG. 6c ) is obviated. Although the vertical NOR strings of FIGS. 6c and6d may not by themselves offer significant areal efficiencies, ascompared to prior art vertical NAND strings (e.g., the NAND strings ofFIG. 1c ), such vertical NOR strings achieve much greater string lengthsthan vertical NAND strings. For example, while vertical NOR strings ofthe present invention may well support strings of length 128 to 512 ormore TFTs in each stack, such string lengths are simply not practicalfor a vertical NAND string, given the serious limitations attendant withseries-connected TFT strings.

Fabrication Process

FIGS. 7a, 7b, 7c and 7d are cross sections of intermediate structuresformed in a fabrication process for a multi-gate NOR string array, inaccordance with one embodiment of the present invention.

FIG. 7a shows a cross section in the Z-Y plane of semiconductorstructure 700, after low resistivity layers 723 p have been formed abovesubstrate 701, in accordance with one embodiment of the presentinvention. In this example, p is an integer between 0 and 31,representing each of 32 word lines. As shown in FIG. 7a , semiconductorstructure 700 includes low resistivity layers 723-0 to 723-31.Semiconductor substrate 701 represents, for example, a P− doped bulksilicon wafer on and in which support circuits for memory structure 700may be formed prior to forming the vertical NOR strings. Such supportcircuits may include both analog and digital logic circuits. Someexamples of such support circuits may include shift registers, latches,sense amplifiers, reference cells, power supply lines, bias andreference voltage generators, inverters, NAND, NOR, Exclusive-OR andother logic gates, input/output drivers, address decoders, includingbit-line and word line decoders, other memory elements, sequencers andstate machines. To provide these support circuits, the building blocksof conventional N-Wells, P-Wells, triple wells (not shown), N±diffusionregions (e.g., region 707-0) and P+ diffusion regions (e.g., region706), isolation regions, low and high voltage transistors, capacitors,resistors, diodes and interconnects are provided, as known to a personskilled in the art.

After the support circuits have been formed in and on semiconductorsubstrate 701, insulating layers 708 are provided, which may bedeposited or grown thick silicon dioxide, for example. In someembodiments, one or more metallic interconnect layers may be formed,including global source line 713-0, which may be provided as horizontallong narrow strips running along a predetermined direction. Globalsource line 713-0 is connected through etched openings 714 to circuitry707 in substrate 701. To facilitate discussion in this detaileddescription, the global source lines are presumed to run along the Xdirection. The metallic interconnect lines may be formed by applyingphoto-lithographical patterning and etching steps on one or moredeposited metal layers. (Alternatively, these metallic interconnectlines can be formed using a conventional damascene process, such as aconventional copper or tungsten damascene process). Thick dielectriclayer 709 is then deposited, followed by planarization usingconventional chemical mechanical polishing (CMP).

Conductor layers 723-0 to 723-31 are then successively formed, eachconductor layer being insulated from the layer underneath it and thelayer above it by an intervening insulating layers 726. In FIG. 7a ,although thirty two conductor layers are indicated, any number of suchlayers may be provided. In practice, the number of conductor layers thatcan be provided may depend on the process technology, such as theavailability of a well-controlled anisotropic etching process thatallows cutting through the multiple conductor layers and dielectricisolation layers 726 there-between. For example, conductor layers 723 pmay be formed by first depositing 1-2 nm thick layer of titanium nitride(TiN), followed by depositing a 10-50 nm thick layer of tungsten (W) ora similar refractory metal, or a silicide such as silicides of nickel,cobalt or tungsten among others, or a salicide, followed by a thin layerof etch-stop material such as aluminum oxide (Al₂O₃). Each conductorlayer is etched in a block 700 after deposition, or is deposited as ablock through a conventional damascene process. In the embodiment shownin FIG. 7a , each successive conductor layer 723 p extends in theY-direction a distance 727 short of (i.e. recessed from) the edge of theimmediately preceding metal layer, so that all conductor layers may becontacted from the top of structure 700 at a later step in the process.However, to reduce the number of masking and etch steps necessary toform the stepped conductors stack of FIG. 7a , it is possible to achieverecessed surfaces 727 simultaneously for multiple conductor layers byemploying other process techniques known to a person skilled in the artthat do not require each individual conductor plane to be separatelymasked and etched to create exposed recessed surfaces 727. After theconductor layer is deposited and etched, the corresponding one ofdielectric isolation layers 726 is then deposited. Dielectric isolationlayers 726 may be, for example, a silicon dioxide of a thickness between15 and 50 nanometers. Conventional CMP prepares the surface of eachdielectric layer for depositing the next conductor layer. The number ofconductor layers in the stack of block 700 corresponds to at least thenumber of memory TFTs in a vertical NOR string, plus any additionalconductor layers that may be used as control gates of non-memory TFTssuch as pre-charge TFTs (e.g., pre-charge TFT 575 of FIG. 5), or ascontrol gates of bit-line access select TFTs (e.g., 585 bit-line accessselect TFT 511 of FIG. 5). The conductor layer deposition and etch stepsand the dielectric layer deposition and CMP process are repeated untilall conductor layers are provided.

Dielectric isolation layer 710 and hard mask layer 715 are thendeposited. Hard mask 715 is patterned to allow etching of conductorlayers 723 p to form long strips of yet to be formed word lines. Theword lines extend in length along the Y direction. One example of amasking pattern is shown in FIG. 6 for word lines 623 p-R, 623 p-L,which includes features such as the extensions in adjacent word linestowards each other at separation 676 and the recesses in each word lineto create the desired curvatures 675. Deep trenches are created byanisotropically etching through successive conductor layers 723 p andtheir respective intervening dielectric insulator layers 726, untildielectric layer 709 at the bottom of conductor layers 723 p is reached.As a large number of conductor layers are etched, a photoresist mask byitself may not be sufficiently robust to hold the desired word linepattern through numerous successive etches. To provide a robust mask,hard mask layer 715 (e.g., carbon) is preferred, as is known to a personof ordinary skill in the art. Etching may terminate at dielectricmaterial 709, or at landing pads 713 on the global source lines, or atsubstrate 701. It may be advantageous to provide an etch-stop barrierfilm (e.g., aluminum oxide) to protect landing pads 713 from etching.

FIG. 7b illustrates, in a cross section in the Z-X plane ofsemiconductor structure 700, etching through successive conductor layers723 p and corresponding dielectric layers 726 to form trenches (e.g.,deep trench 795), which reach down to dielectric layer 709, according toone embodiment of the present invention. In FIG. 7b , conductor layers723 p are anisotropically etched to form conductor stacks 723 p-R and723 p-L, which are separated from each other by deep trench 795. Thisanisotropic etch is a high aspect-ratio etch. To achieve the bestresult, etch chemistry may have to be alternated between conductormaterial etch and dielectric etch, as the materials of the differentlayers are etched through, as in known to a person skilled in the art.The anisotropy of the multi-step etch is important, as undercutting ofany of the layers should be avoided, so that a resulting word line atthe bottom of a stack would have approximately the same conductor widthand trench spacing as the corresponding width and spacing of a word linenear or at the top of the stack. Naturally, the greater the number ofconductor layers in the stack, the more challenging it becomes tomaintain a tight pattern tolerance through the numerous successiveetches. To alleviate the difficulty associated with etching through, forexample, 64 or 128 or more conductor layers, etching may be conducted insections of, say, 32 layers each. The separately etched sections canthen be stitched together, as taught, for example, in the Kim referencementioned above.

Etching through multiple conductor layers 723 p of conductor material(e.g., tungsten or other refractory materials) is much more difficultand time-consuming than etching of the intervening insulating layers726. For that reason, an alternative process may be adopted thateliminates the need for multiple etches of conductor layers 723 p. Thatprocess, well known to a person skilled in the art, consists of firstsubstituting sacrificial layers of a readily etchable material in placeof conductor layers 723 p of FIG. 7b . For example, insulating layers726 can be silicon dioxide and sacrificial layers (occupying the spacesshown as 723 p in FIG. 7b ) can be silicon nitride or another fastetching dielectric material. Deep trenches are then etchedanisotropically through the ONON (Oxide-Nitride-Oxide-Nitride)alternating dielectric layers to create tall stacks of the dualdielectrics. At a later step in the manufacturing process flow (to bedescribed below), these stacks are supported by active vertical stripsof polysilicon, allowing the sacrificial layers to be etched away,preferably through selective chemical or isotropic etch. The cavitiesthus created are then filled through conformal deposition of theconductor material, resulting in conductor layers 723 p separated byintervening insulating layers 726.

After the structure of FIG. 7b is formed, charge-trapping layers 734 andpolysilicon layers 730 are then deposited in succession conformally onthe vertical sidewalls of the etched conductor word line stacks. A crosssection in the Z-X plane of the resulting structure is shown in FIG. 7c. As shown in FIG. 7c , charge-trapping layers 734 are formed, forexample, by first depositing blocking dielectric 732 a, between 5 to 15nanometers thick and consisting of a dielectric film of a highdielectric constant (e.g., aluminum oxide, hafnium oxide, or somecombination silicon dioxide and silicon nitride). Thereafter,charge-trapping material 732 b is deposited to a thickness of 4 to 10nanometers. Charge-trapping material 732 b may be, for example, siliconnitride, silicon-rich oxynitride, conductive nanodots embedded in adielectric film, or thin conductive floating gates isolated fromadjacent TFTs sharing the same vertical active strip. Charge-trapping732 b may then be capped by a deposited conformal thin tunnel dielectricfilm in the thickness range of 2 to 10 nanometers (e.g., a silicondioxide layer, or a silicon oxide-silicon nitride-silicon oxide (“ONO”)triple-layer). The storage element formed out of charge-trapping layers734 may be any one of SONOS, TANOS, nanodot storage, isolated floatinggates or any suitable charge-trapping sandwich structures known to aperson of ordinary skill in the art. The combined thickness ofcharge-trapping layers 734 is typically between 15 and 25 nanometers.

After deposition of charge-trapping layer 734, contact openings are madeat the bottom of trench 795, using a masking step and by anisotropicallyetching through charge-trapping layers 734 and dielectric layer 709 atthe bottom of trench 795, stopping at bottom global source line landingpad 713 for the source supply voltage V_(ss) (see, FIG. 7b ), or atglobal bit line voltage V_(bl) (not shown), or at P+ region 706 forcontact to a back bias supply voltage V_(bb) (see, FIG. 7c ). In someembodiments, this etch step is preceded by a deposition of an ultra-thinfilm of polysilicon (e.g. 2 to 5 nanometers thick) to protect thevertical surfaces of tunnel dielectric layer 732 c during thecontact-opening etch of charge-trapping material 734 at the bottom oftrench 795. In one embodiment, each global source line is connected onlyto alternate ones in a row of vertical NOR string pairs. For example, inFIG. 5, for odd address word lines, electrical contacts (e.g., contactopening 557) are etched to connect the N+ doped local source lines(e.g., local source line 555 in FIG. 5) to global source line 513-1.Likewise, for even address word lines, electrical contacts are etched toconnect the N+ doped local source lines in the row of vertical NORstring pairs to global source line 513-2 (not shown in FIG. 5). In theembodiment employing virtual V_(ss) through parasitic capacitor C (i.e.,capacitors 560 in FIG. 5) the step of etching through charge trappinglayer 734 at the bottom of trench 795 may be skipped.

Thereafter, polysilicon thin film 730 is deposited to a thicknessranging between 5 and 10 nanometers. In FIG. 7c , polysilicon thin film730 is shown on the opposite sidewalls of trench 795, labeledrespectively 730R and 730L. Polysilicon thin film 730 is undoped orpreferably doped P− with boron, at a doping concentration typically inthe range of 1×10¹⁶ per cm³ to 1×10¹⁷ per cm³, which allows a TFT to beformed therein to have an enhancement native threshold voltage. Trench795 is sufficiently wide to accommodate charge-trapping layers 734 andpolysilicon thin film 730 on its opposing sidewalls. Following thedeposition of polysilicon 730, the sacrificial layers in the stackdescribed above are etched away and the cavities thus formed are filledwith the conformally deposited conductor layers 723 p (FIG. 7c ).

As shown in FIG. 7b , trench 795 extends along the Y direction. Afterformation of isolated word line stacks 723 p-L and 723 p-R, in oneexample semiconductor structure 700 may have 16,000 or more side-by-sideword line stacks, each serving as control gates for 8,000 or more activecolumns to be formed along the length of each stack, or 16,000 TFTs(8,000 TFTs on each side of the stack). With 64 word lines in eachstack, 16 billion TFTs may eventually be formed in each of suchmulti-gate vertical NOR string array. If each TFT stores two data bits,such a multi-gate vertical NOR string array would store 32 gigabits ofdata. Approximately 32 such multi-gate vertical NOR string arrays (plusspare arrays) may be formed on a single semiconductor substrate, therebyproviding a 1-terabit integrated circuit chip.

FIG. 7d is a cross section view in the X-Y plane of the top surface ofthe structure of FIG. 7c in one embodiment. Nestled between word lines723 p-L and 723 p-R are the two sidewalls 730L and 730R of the verticaldeposited P− doped polysilicon structure (i.e., an active column). Thedeep void 740 between sidewalls 730L and 730R may be filled with afast-etching insulating dielectric material (e.g., silicon dioxide orliquid glass or carbon doped silicon oxide). The top surface may then beplanarized using conventional CMP. A photolithographic step then exposesopenings 776 and 777, which is followed by a high aspect-ratio selectiveetching to excavate the fast-etching dielectric material in exposedareas 776 and 777 all the way down to the bottom of trench 795. A hardmask may be required in this etching step to avoid excessive patterndegradation during etch. The excavated voids are then filled with anin-situ N+ doped polysilicon. The N+ dopants diffuse into the very thinlightly doped active polysilicon pillars 730L and 730R within theexposed voids to make them N+ doped. Alternatively, prior to filling thevoids with the in-situ N+ doped polysilicon the lightly dopedpolysilicon inside the voids can be etched away through a briefisotropic plasma etch or selective wet etch. CMP or top surface etchingthen removes the N+ polysilicon from the top surface, leaving tall N+polysilicon pylons in areas 754 (N+) and 755(N+). These N+ pylons formthe shared vertical local source line and the shared vertical local bitline for the TFTs in the resulting vertical NOR strings.

Next, a dielectric isolation layer is deposited and patterned usingphotolithographic masking and etching steps. The etching step openscontacts for connecting the vertical local bit lines to the horizontalglobal bit lines (e.g., contacts 657-1 to strings at odd addresses and657-2 to strings at even addresses, as shown in FIG. 6). A lowresistivity metal layer (e.g., tungsten) is deposited. The depositedmetal is then patterned using photolithographic and etching steps toform global bit-lines (e.g., global word line 614-1 or GBL1 for stringsat odd addresses, and global bit line 614-2 (GBL2) for strings at evenaddresses, as shown in FIG. 6). Alternatively, the global bit lines maybe formed using conventional copper damascene process. All global bitlines, as well as all metal layers 723 p of the word line stacks (FIG.7a ) are connected by etched vias to word line and bit-line decoding andsensing circuits in the substrate, as is known to a person skilled inthe art. Switch and sensing circuits, decoders and reference voltagesources can be provided to global bit lines and global word lines,either individually or shared by several ones of the bit lines and wordlines.

In some embodiments, bit line access select transistors (511 in FIG. 5)and their associated control gate word lines (e.g., word lines 585 inFIG. 5) are formed as isolated vertical N+P-N+ transistors, as known toa person skilled in the art, to selectively connect odd and even globalbit lines (e.g., bit lines 614-1 and 614-2 in FIG. 6a ) to vertical NORstrings at alternate odd and even addresses (e.g., local bit lines 657-1and 657-2, respectively, in FIG. 6a ).

Read Operation

Because the TFTs of a vertical NOR string are connected in parallel, inall embodiments of the current invention, all TFTs in an active column(including an active column having formed thereon a vertical NOR stringpair) should preferably be in enhancement mode—i.e., each TFT shouldhave a positive gate-to-source threshold voltage—so as to suppressleakage currents during a read operation between the shared local sourceline and the shared local bit line (e.g., local bit line 455 and localsource line 454 shown in FIG. 4c ). Enhancement mode TFTs are achievedby doping the channel regions (e.g., P− S channel region 756 of FIG. 7c) with boron in a concentration typically between 1×10¹⁶ and 1×10¹⁷ percm³, targeting for a native TFT threshold voltage of around 1V. Withsuch TFTs, all unselected word lines in the vertical NOR string pair ofan active column may be held at 0V. Alternatively, the read operationmay raise the voltage on the shared local N+ source line (e.g., localsource line 455 of FIG. 4c ) to around 1.5V, while raising the voltageon the shared local N+ drain line (e.g., local bit line 454) to around2V and holding all unselected local word lines at 0V. Such aconfiguration is equivalent to setting the word line to −1.5V withrespect to the source, thereby suppressing leakage current due to TFTsthat are in slightly depleted threshold voltage, which occurs, forexample, if the TFTs are slightly over-erased.

After erasing the TFTs of a vertical NOR string, a soft programmingoperation may be required to shift any TFT in the vertical NOR stringthat is over-erased (i.e., now having a depletion mode thresholdvoltage) back to an enhancement mode threshold voltage. In FIG. 5, anoptional connection 556 is shown by which P− channel is connected toback bias voltage 506 (V_(bb),) (also shown as body connection 456 inFIG. 4c ). A negative voltage may be used for V_(bb) to modulate thethreshold voltage of the TFTs in each active column to reducesubthreshold leakage currents between the shared N+ source and theshared N+ drain/local bit line. In some embodiments, a positive V_(bb)voltage can be used during an erase operation to tunnel-erase TFTs whosecontrol gates are held at 0V.

To read the data stored in a TFT of a vertical NOR string pair, all TFTson both vertical NOR strings of the vertical NOR string pair areinitially placed in the “off” state by holding all word lines in themulti-gate NOR string array at 0V. The addressed vertical NOR string caneither share a sensing circuit among several vertical NOR strings alonga common word line through use of decoding circuitry. Alternatively,each vertical NOR string may be directly connected through a globalbit-line (e.g., GBL1 of FIG. 4c ) to a dedicated sensing circuit. In thelatter case, one or more vertical NOR strings sharing the same word lineplane may be sensed in parallel. Each addressed vertical NOR string hasits local source line set at V_(ss)˜0V, either through its hard-wiredglobal source line (e.g., GSL1 in FIG. 4c ) as shown schematically inFIG. 8a , or as a virtual V_(ss)˜0V through a pre-charge transistor(e.g., pre-charge transistor 470 in FIG. 4c or transistor 317 in FIG. 3c) which momentarily transfers V_(bl)˜0V to parasitic capacitance C(e.g., capacitor 460 or capacitor 360) of floating local source line 455or 355) during the pre-charge, as shown schematically in FIG. 8 b.

Immediately after turning off pre-charge transistor 470, the local bitline (e.g., local bit line 454 of FIG. 4c ) is set at V_(bl)˜2V throughthe bit line access select transistor (e.g., bit line access selecttransistor 411 of FIG. 4c or access select transistor 511 in FIG. 5).V_(bl)˜2V is also the voltage at the sense amplifiers for the addressedvertical NOR strings. At this time, the addressed word line is raised insmall incremental voltage steps from 0V to typically about 6V, while allthe un-selected word lines at both the odd address TFTs and the evenaddress TFTs of the vertical NOR string pair remain at 0V. In theembodiment of hard-wired V_(ss) of FIG. 8a , the addressed TFT has beenprogrammed in one example to a threshold voltage of 2.5V, therefore thevoltage V_(bl) at local bit line LBL begins to discharge through theselected TFT towards the 0V of the local source line (V_(ss)) as soon asits WLs exceeds 2.5V, thus providing a voltage drop (shown by the dashedarrow in FIG. 8a ) that is detected at the sense amplifier serving theselected global bit line. In the embodiment of the virtual V_(ss) ofFIG. 8b , pre-charge transistor word line WL_(CHG) momentarily is turnedon to pre-charge floating local source line LSL to 0V at the start ofthe read sequence. Then, selected word line WLs goes through itsincremental voltage steps, and as soon as it exceeds the programmed2.5V, the selected TFT momentarily dips the voltage on its local bitline from its V_(bl)˜2V. This voltage dip (shown by the dashed arrow inFIG. 8b ) is detected by the sense amplifier of the global bit lineconnected to the selected local bit line. There are other alternativeschemes to correctly read the programmed threshold voltage of theselected TFT as known to a person skilled in the art. It should bepointed out that for the embodiments relying on parasitic capacitance Cto temporarily hold virtual voltage V_(ss), the higher the verticalstack the bigger is capacitance C and therefore the longer is the holdtime and the greater is the read signal presented to the selected senseamplifier. To further increase C it is possible to add in one embodimentone or more dummy conductors in the vertical string whose primarypurpose is to increase capacitance C.

In the case of an MLC implementation (i.e., a “multi-level cell”implementation, in which each TFT stores more than one bit), theaddressed TFT may have been programmed to one of several voltages (e.g.,1V (erased state), 2.5V, 4V or 5.5V). The addressed word line WLs israised in incremental voltage steps until conduction in the TFT isdetected at the sense amplifier. Alternatively, a single word linevoltage can be applied (e.g., ˜6 volts), and the rate of discharge ofthe local bit line LBL (V_(bl)) can be compared with the rates ofdischarge from several programmable reference voltages representative ofthe voltage states of the stored multi-bit. This approach can beextended for a continuum of states, effectively providing analogstorage. The programmable reference voltages maybe stored in dedicatedreference vertical NOR strings located within the multi-gate verticalNOR string array, so that the characteristics during read, program, andbackground leakage are closely tracked. In a vertical NOR string pair,only the TFTs on one of the two vertical NOR strings can be read in eachread cycle; the TFTs on the other vertical NOR string are placed in the“off” state (i.e., all word lines at 0V). During a read cycle, as onlyone of the TFTs in a vertical NOR string is exposed to the readvoltages, read disturb conditions are essentially absent.

In one example of an embodiment of this invention, 64 TFTs and one ormore pre-charge TFTs may be provided on each vertical NOR string of avertical NOR string pair. Each word line at its intersection with thelocal vertical N+ source line pillar forms a capacitor (see, e.g.,capacitor 660 of FIG. 6a ). A typical value for such a capacitor may be,for example, 1×10⁻¹⁸ farads. Including all the capacitors in bothvertical NOR strings of a vertical NOR string pair, the overalldistributed capacitance C totals approximately 1×10⁻¹⁶ farads, which issufficient for a local source line to preserve a pre-charged sourcevoltage (V_(ss)) during a read cycle, which is completed in typicallyless than a microsecond immediately following the pre-charge operation.The charging time through bit-line access select transistors 411 andpre-charge TFT 470 is in the order of a few nanoseconds, thus thecharging time does not add noticeably to the read latency. Reading froma TFT in a vertical NOR string is fast, as the read operation involvesconduction in only one of the TFTs in the vertical NOR string, unlikethe read operation on a NAND string, in which many TFTs connected inseries are required to be conducting.

There are two major factors contributing to the read latency of verticalNOR strings of the current invention: (a) the RC time delay associatedwith resistance R_(bl) and capacitance C_(bl) of a global bit line(e.g., GBL 614-1 in FIG. 6a ), and (b) the response time of a senseamplifier to a voltage drop V_(bl) on the local bit line (e.g., LBL-1)when the addressed TFT begins conducting. The RC time delay associatedwith a global bit line serving, for example, 16,000 vertical NOR stringsis of the order of a few tens of nanoseconds. The read latency forreading a TFT of a prior art vertical NAND string (e.g., the NAND stringof FIG. 1b ) is determined by the current through 32 or moreseries-connected TFTs and select transistors discharging capacitanceC_(bl) of the global bit line. By contrast, in a vertical NOR string ofthe present invention, the read current discharging C_(bl) is providedthrough just the one addressed transistor (e.g., transistor 416L of FIG.4a ) in series with bit line access select transistor 411, resulting ina much faster discharge of the local bit line voltage (V_(bl)). As aresult, a much lower latency is achieved.

In FIG. 4c , when one TFT (e.g., TFT 416L in the vertical NOR string 451b) is read at a time, all other TFTs in either vertical NOR string 451 aand 451 b of vertical NOR string pair 491 are held in their “off”states, their word lines being held at 0V. Even though TFT 416R invertical NOR string 452 a of vertical NOR string pair 492 shares wordline W31 with TFT 416L, TFT 416R may be read simultaneously with TFT416L because vertical NOR string 452 a is served by global bit line414-2, while vertical NOR string 451 b is served by global bit line414-1. (FIGS. 6a and 6b illustrate how global bit lines 614-1 and 614-2serve adjacent vertical NOR string pairs).

In one embodiment, a word line stack includes 32 or more word linesprovided in 32 planes. In one multi-gate vertical NOR string array, eachplane may include 8000 word lines controlling 16,000 TFTs, each of whichmay be read in parallel through 16,000 global bit lines, provided thateach bit line is connected to a dedicated sense amplifier.Alternatively, if several global bit lines share a sense amplifierthrough a decode circuit, the 16000 TFTs are read over severalsuccessive read cycles. Reading in parallel a massive number ofdischarging TFTs can cause a voltage bounce in the ground supply(V_(ss)) of the chip, which may result in read errors. However, anembodiment that uses the pre-charged parasitic capacitor C in the localsource line (i.e., providing a virtual source voltage (V_(ss)) forvertical NOR string) has a particular advantage in that such groundvoltage bounce is eliminated. This is because the virtual sourcevoltages in the vertical NOR strings are independent and are notconnected to the ground supply of the chip.

Program (Write) and Program-Inhibit Operations.

Programming of an addressed TFT may be achieved by tunneling—eitherdirect tunneling or Fowler-Nordheim tunneling,—of electrons from thechannel region of the TFT (e.g., channel region 430L shown in FIG. 4b )to the charge-trapping layer (e.g., charge trapping layer 434) when ahigh programming voltage is applied between the selected word line(e.g., word line 423 p-R) and the active channel region (e.g., activechannel region 456 in FIG. 4a ). As tunneling is highly efficient,requiring very little current to program a TFT, parallel programming oftens of thousands of TFTs may be achieved at low power dissipation.Programming by tunneling may require, for example, a 20V,100-microsecond pulse. Preferably, the programming is implementedthrough a succession of shorter duration stepped voltage pulses,starting at around 14V and going as high as approximately 20V. Steppedvoltage pulsing reduces electrical stress across the TFT and avoidsovershooting the intended programmed threshold voltage.

After each programming high-voltage pulse the addressed transistor isread to check if it has reached its target threshold voltage. If thetarget threshold voltage has not been reached, the next programmingpulse applied to the selected word line is incremented typically by afew hundred millivolts. This program-verify sequence is repeatedlyapplied to the one addressed word line (i.e., a control gate) with 0Vapplied to the local bit line (e.g., local bit line 454 of FIG. 4a ) ofthe active column (e.g., column 430L of FIG. 4b ). At these programminghigh word line voltages, TFT 416L's channel region is inverted and isheld at 0V, so that electrons tunnel into the charge storage layer ofTFT 416L. When the read sensing indicates that the addressed TFT hasreached its target threshold voltage, the addressed TFT must beinhibited from further programming, while other TFTs sharing the sameword line may continue programming to their higher target thresholdvoltages. For example, when programming TFT 416L in vertical NOR string451 b, programming of all other TFTs in vertical NOR strings 451 b and451 a must be inhibited by keeping all their word lines at 0V.

To inhibit further programming or TFT 416L once it has reached itstarget threshold voltage, a half-select voltage (i.e., approximately10V) is applied to local bit line 454. With 10V being placed in thechannel region and 20V being placed on the control gate, only net 10V isapplied across the charge trapping layer, therefore the Fowler-Nordheimtunneling current is insignificant and no meaningful further programmingtakes place on TFT 416L during the remaining sequence of stepped pulsevoltages up to the maximum 20V. By raising the local bit line 454 to 10Vwhile continuing to increment the programming voltage pulses on wordline WL31, all TFTs on vertical NOR strings sharing the same selectedword line are programmed correctly to their higher target thresholdvoltages. The sequence of “program-read-program inhibit” isindispensable for correctly programming tens of thousands TFTs inparallel to their various target threshold voltage states in multilevelcell storage. Absent such program inhibit of individual TFTsover-programming may cause overstepping or merging with the thresholdvoltage of the next higher target threshold voltage state. Although TFT416R and TFT 416L share the same word line, they belong to differentvertical NOR string pairs 452 and 451. It is possible to program bothTFT 416L and TFT 416R in the same programming pulsed voltage sequence,as their respective bit line voltages are supplied through GBL1 and GBL2and are independently controlled. For example, TFT 416L can continue tobe programmed while TFT 416R can be inhibited from further programmingat any time. These program and program-inhibit voltage conditions can bemet because vertical NOR strings 451 a and 451 b of vertical NOR stringpair 491 are controlled by separate word lines 423 p-L and 423 p-Rrespectively, and the voltage on each local bit line can be setindependently from all other vertical NOR string pairs. Duringprogramming, any unselected word line within an addressed word linestack or within unaddressed word line stacks can be brought to 0V,half-select 10 volts, or floated. In the embodiment where global sourceline (e.g., GSL1 of FIG. 4c ) is accessed through a source access selecttransistor (not shown in FIG. 4c ), the access select transistor is offduring programming, resulting in the voltage on local source line 455following the voltage on local bit line 454 during program and programinhibit. The same is true for the embodiment where the voltage on thelocal source line is provided by its parasitic capacitance C representedby capacitor 460 in FIG. 4c . In the embodiment of FIG. 4c , where thereis a global source line but not a source access select transistor, thevoltage applied to the global source line 413-1 of the addressed stringshould preferably track the voltage of the addressed global bit line414-1 during program and program-inhibit.

Each of the incrementally higher voltage programming pulses is followedby a read cycle to determine if TFTs 416L and 416R have reached theirrespective target threshold voltage. If so, the drain, source and bodyvoltages are raised to 10V (alternatively, these voltages are floated toclose to 10V) to inhibit further programming, while word line WL31continues to program other addressed TFTs on the same plane that havenot yet attained their target threshold voltages. This sequenceterminates when all addressed TFTs have been read-verified to becorrectly programmed. In the case of MLC, programming of one of themultiple threshold voltage states can be accelerated by setting eachaddressed global bit line to one of several predetermined voltages(e.g., 0V, 1.5V, 3.0V, or 4.5V, representing the four distinct states ofthe 2-bit data to be stored), and then applying the stepped programmingpulses (up to around 20V) to word line WL31. In this manner, theaddressed TFT receives a predetermined one of the effective tunnelingvoltages (i.e., 20, 18.5, 17, and 15.5 volts, respectively), resultingin one of predetermined threshold voltages being programmed into a TFTin a single programming sequence. Fine programming pulses may besubsequently provided at the individual TFT level.

Accelerated Whole-Plane Parallel Programming

Because of the parasitic capacitance C intrinsic to every local sourceline in a multi-gate vertical NOR string array, all local source linesin a multi-gate vertical NOR string array can have 0V (for program) or10V (for inhibit) momentarily placed (e.g., through global bit line GBL1and bit line access string select transistor 411 and pre-chargetransistor 470) on all vertical NOR strings in advance of applying thehigh voltage pulsing sequence. This procedure may be carried out byaddressing the word line planes plane-by-plane. For each addressed wordline plane, the programming pulsing sequence may be applied to many orall word lines on the addressed word line plane, while holding all wordlines on the other word line planes at 0V, so as to program in parallela large number of TFTs on the addressed plane, followed by individualread-verify, and where necessary, resetting the local source line of aproperly programmed TFT into program-inhibit voltage. This approachprovides a significant advantage, as programming time is relatively long(i.e., around 100 microsecond), while pre-charging all local source linecapacitors or read-verifying all TFTs sharing the addressed word lineplane is more than 1,000 times faster. Therefore, it pays to parallelprogram as many TFTs as possible in each word line plane. Thisaccelerated programming feature provides even greater advantage in MLCprogramming which is considerably slower than single bit programming.

Erase Operation

For some charge-trapping materials, the erase operation is performed byreverse-tunneling of the trapped charge, which can be rather slow,sometimes requiring tens of milliseconds of 20V or higher pulsing.Therefore, the erase operation may be implemented at the vertical NORstring array level (“block erase”), often performed in the background. Atypical vertical NOR string array may have 64 word line planes, witheach word line plane controlling, for example, 16,384×16,384 TFTs, for atotal of approximately seventeen billion TFTs. A one-terabit chip maytherefore include approximately 30 such vertical NOR string arrays, iftwo bits of data are stored on each TFT. In some embodiments, blockerase may be carried out by applying around 20V to the P− channel sharedby all TFTs in a vertical NOR string (e.g., body connection 456 in FIG.4c and contact 556 in FIG. 5), while holding all word lines in the blockat 0V. The duration of the erase pulse should be such that most TFTs inthe block are erased to a slight enhancement mode threshold voltage,i.e., between zero and one volt. Some TFTs will overshoot and be erasedinto depletion mode (i.e., a slightly negative threshold voltage). Asoft programming may be required to return the over-erased TFTs backinto a slight enhancement mode threshold voltage after the terminationof the erase pulses, as part of the erase command Vertical NOR stringsthat may include one of more depletion mode TFTs that cannot beprogrammed into enhancement mode may have to be retired, to be replacedby spare strings.

Alternatively, rather than providing the erase pulses to the body (i.e.,the P− layer), the local source lines and the local bit lines (e.g.,local source line 455 and local bit line 454 in FIG. 4c ) on allvertical NOR string pairs in the vertical NOR string array are raised toaround 20V, while holding all word lines on all word line planes at 0Vfor the duration of the erase pulse. This scheme requires that theglobal source line and the global bit line select decoders employ highvoltage transistors that can withstand the 20V at their junctions.Alternatively, all TFTs sharing an addressed word line plane can beerased together by applying −20V pulses to all word lines on theaddressed plane, while holding word lines on all other planes at 0V. Allother voltages in the vertical NOR string pairs are held at 0V. Thiswill erase only the X-Y slice of all TFTs touched by the one addressedplane of word lines.

Semi Non-Volatile NOR TFT Strings

Some charge-trapping materials (e.g., oxide-nitride-oxide or “ONO”)suitable for use in the vertical NOR string have long data retentiontime, typically in the order of many years, but relatively low endurance(i.e., performance degrades after some number of write-erase cycles,typically of the order of ten thousand cycles or less). However, in someembodiments one may select charge-trapping materials that store chargefor much reduced retention times, but with much increased endurances(e.g., retention times in order of minutes or hours, endurance in theorder of tens of millions of write-erase cycles). For example, in theembodiment of FIG. 7c , the tunnel dielectric layer 732 c, typically a6-8 nanometer layer of SiO₂, can be reduced in thickness to around 2nanometers, or be replaced by another dielectric material (e.g., SiN) ofsimilar thickness. The much thinner dielectric layer makes possible theuse of modest voltages to introduce electrons by direct tunneling (asdistinct from Fowler-Nordheim tunneling, which requires a highervoltage) into the charge-trapping layer, where they will be trapped froma few minutes to a few hours or days. Charge-trapping layer 732 b can besilicon nitride, conductive nanodots dispersed in a thin dielectricfilm, or a combination of other charge-trapping films, includingisolated thin floating gates. Blocking layer 732 a can be silicondioxide, aluminum oxide, hafnium oxide, silicon nitride, a highdielectric constant dielectric, or any combination thereof. Blockinglayer 732 a blocks electrons in charge-trapping layer 732 b fromescaping to the control gate word line. Trapped electrons willeventually leak out back into active region 730R, either as a result ofthe breakdown of the ultra-thin tunnel dielectric layer, or by reversedirect tunneling. However, such loss of trapped electrons is relativelyslow. One may also use other combinations of charge storage materials,resulting in a high endurance but low retention “semi-volatile” storageTFT that requires periodic write or read refresh operations to replenishthe lost charge. Because the vertical NOR strings of the presentinvention have a relatively fast read access (i.e. low latency), theymay be used in some applications that currently require the use ofdynamic random access memories (DRAMs). The vertical NOR strings of thepresent invention have significant advantages over DRAMs, having a muchlower cost-per-bit, as DRAMs cannot be built in three dimensionalstacks, and having a much lower power dissipation, as the refresh cyclesneed only be run approximately once every few minutes or every fewhours, as compared to every few milliseconds required to refresh DRAMs.The three-dimensional semi-volatile storage TFTs of the presentinvention are achieved by selecting an appropriate material, such asthose discussed above, for the charge-trapping material and byappropriately adapting the program/read/program-inhibit/erase conditionsand incorporating the periodic data refreshes.

NROM/Mirror Bit NOR TFT Strings

In another embodiment of the current invention, the vertical NOR stringsmay be programmed using a channel hot-electron injection approach,similar to that which is used in two-dimensional NROM/Mirror Bittransistors, known to a person skilled in the art. Using the embodimentof FIG. 4a as an example, programming conditions for channelhot-electron injection may be: 8V on control gate 423 p, 0V on localsource line 455 and 5V on local drain line 454. Charge representing onebit is stored in the charge storage layer at one end of channel region456 next to the junction with local bit line 454. By reversing polarityof local source line 455 and local bit line 454, charge representing asecond bit is programmed and stored in the charge storage layer at theopposite end of channel region 456 next to the junction with localsource line 455. Reading both bits requires reading in reverse order ofthe programming, as is well known to those skilled in the art. Channelhot-electron programming is much less efficient than programming bydirect tunneling or Fowler-Nordheim tunneling and therefore it does notlend itself to the massively parallel programming possible withtunneling. However, each TFT has twice the bit density, making itattractive for applications such as archival memory. Erase for the NROMTFT embodiment can be achieved by employing the conventional NROM erasemechanism of band to band tunneling-induced hot-hole injection toneutralize the charge of the trapped electrons: apply −5V on the wordline, 0V to local source line 455 and 5V to local bit line 454.Alternatively, the NROM TFT can be erased by applying a high positivesubstrate voltage V_(bb) to body region 456 with the word line at 0V.Because of the high programming current attendant to channel hotelectron injection programming, all embodiments of vertical NROM TFTstrings must employ hard-wired local source line and local bit line,such as in the embodiments of FIGS. 3a and 6 c.

The above detailed description is provided to illustrate specificembodiments of the present invention and is not intended to be limiting.Numerous variations and modification within the scope of the presentinvention are possible. The present invention is set forth in theaccompanying claims.

I claim:
 1. A memory array formed above a substantially planar surfaceof a semiconductor substrate, the semiconductor substrate havingcircuitry formed therein, comprising a plurality of NOR-type memorystrings, wherein each NOR-type memory string comprises a plurality ofthin-film storage transistors sharing a common source region and acommon drain region, the common source region and the common drainregion each being a column of semiconductor material of a firstconductivity type extending along a first direction that issubstantially perpendicular to the planar surface, wherein (i) eachthin-film storage transistor has a charge-trapping region, a channelregion and a gate electrode that is isolated from the channel region bythe charge-trapping region, and (ii) the gate electrodes of eachthin-film storage transistors are each part of a corresponding one of afirst set of conductors, each conductor extending along a seconddirection substantially parallel to the planar surface.
 2. The memoryarray of claim 1, wherein the charge-trapping regions of the thin-filmstorage transistors are each a portion of a layer of charge-trappingmaterial extending along the first direction.
 3. The memory array ofclaim 2, wherein the channel regions of the thin-film storagetransistors are each a portion of a layer of semiconductor material of asecond conductivity type opposite the first conductivity type.
 4. Thememory array of claim 1, wherein the NOR-type memory strings are arrayedboth along the second the direction and along a third direction alsosubstantially parallel the planar surface and substantially orthogonalto the second direction.
 5. The memory array of claim 1, wherein thecommon source region of each NOR-type memory string is electricallyfloating relative to the circuitry formed in the semiconductorsubstrate, except when one or more of the channel regions of thethin-film transistors of the NOR-type memory string is renderedconducting.
 6. The memory array of claim 1, wherein each of theconductors is shared by gate electrodes of thin-film transistors in aplurality of the NOR-type memory strings of the memory array.
 7. Thememory circuit of claim 5, further comprising a second set of conductorsextending along the third direction, wherein a first group of the secondset of conductors each connect a portion of the circuitry in thesemiconductor substrate to the common drain regions of selected one ormore of the NOR-type memory strings in the memory array.
 8. The memoryarray of claim 7, wherein the second set of conductors are formedbetween the memory array and the planar surface.
 9. The memory array ofclaim 7, wherein the second set of conductors are formed above thememory array.
 10. The memory array of claim 8, wherein the first groupof the second set of conductors connect to the common drain regions ofthe selected memory arrays through select transistors.
 11. The memoryarray of claim 8, wherein a second group of the second set of conductorsconnect corresponding ones of the first set of conductors throughconductor-filled vias.
 12. The memory array of claim 1, wherein eachthin-film storage transistor has a data retention time shorter than ayear and a program/erase cycle endurance greater than 10,000program/erase cycles.
 13. The memory array of claim 1, wherein eachthin-film storage transistor has a native enhancement mode thresholdvoltage.
 14. The memory array of claim 1, wherein the circuitry of thesemiconductor substrate comprises voltage sources for providingpredetermined voltages for memory operations.
 15. The memory circuit ofclaim 14, wherein the predetermined voltages comprise voltages forprogram, program-inhibit, reading and erasing voltages.
 16. The memoryarray of claim 1, wherein each thin-film storage transistor has avariable threshold voltage that is set using Fowler-NordheimtunnelinRM-002-D4 g or direct tunneling.
 17. The memory circuit of claim16, wherein the variable threshold voltage is set to a levelcorresponding to one of two or more charge states.
 18. The memorycircuit of claim 1 wherein, in each NOR-type memory string, the commonsource region and the common drain region each include a dopant-blockinglayer adjacent the channel region of each thin-film storage transistor.19. The memory array of claim 18, wherein the dopant diffusion-blockinglayer comprises a dielectric material that is less than three nanometersthick.
 20. The memory array of claim 1, wherein the channel region ofeach thin-film storage transistor is electrically connected to thesemiconductor substrate.
 21. The memory circuit of claim 20, wherein thechannel region of each thin-film storage transistor is connected to thesemiconductor substrate by a pillar of semiconductor material of thefirst conductivity type.
 22. The memory array of claim 20, wherein thesemiconductor substrate provides the channel region of each thin-filmstorage transistor a predetermined back bias voltage that suppressessub-threshold leakage during read operations.
 23. The memory array ofclaim 1, wherein the channel region of each thin-film storage transistorhas a length that is sufficiently short to effectuate erase throughlateral hopping conduction and tunneling out of stored charge.
 24. Thememory array of claim 1, wherein the first set of conductors eachcomprise one of N⁺ doped polysilicon, P⁺ doped Polysilicon, a refractorymetal of a high work function with respect to silicon dioxide, orsilicides or polycides.
 25. The memory array of claim 1, wherein thecharge-trapping region comprises one or more layers of silicon nitrideor a bandgap engineered oxide-nitride-oxide dielectric layer.
 26. Thememory array of claim 1, wherein each thin-film storage transistor ofeach NOR-type memory string is individually addressable for programming,programming-inhibiting, erasing or reading operations.
 27. The memoryarray of claim 1, wherein the thin-film transistors are non-volatile orquasi-volatile.
 28. The memory array of claim 1 wherein, during a readoperation, an intrinsic capacitor of the common source region of eachNOR-type memory string is charged to a virtual ground voltage and anintrinsic capacitor of the common drain region of the NOR-type memorystring is charged to a read-sensing voltage,
 29. The memory array ofclaim 28, wherein the gate electrode is raised to predetermined voltagesto allow sensing of a threshold voltage of a selected thin-film storagetransistor, while the gate electrodes of all other thin-film selectedtransistors of the NOR-type memory string are held at a non-conductingstate.
 30. The memory array of claim 28 wherein, during a programmingoperation, the intrinsic capacitors of the common source region and thecommon drain region are each momentarily pre-charged to a virtual groundvoltage.
 31. The memory array of claim 1, wherein one or moreprogramming voltage pulses are applied selected one or more of the firstset of conductors to initiate efficient Fowler-Nordheim tunneling ordirect tunneling of charge from the channel region, the common sourceregion and the common drain region to the charge-trapping region, whileall other ones of the first set of conductors are held at a voltage thatinhibits initiation of efficient Fowler-Nordheim tunneling or directtunneling of charge from their channel regions, the common source, andthe common drain region to the charge-trapping layer.
 32. The memoryarray of claim 1, wherein the thin-film storage transistors of multipleNOR-type memory strings are erased in a single operation.
 33. The memoryarray of claim 1, wherein one or more of the NOR-type memory stringsserve as a reference memory string.
 34. The memory array of claim 33,the circuitry in the semiconductor substrate comprise a differentialsense amplifier which compares a signal received from a thin-filmstorage transistor in one of the NOR-type memory string and a signalreceived from a corresponding thin-film storage transistor of thereference NOR string.
 35. The memory array of claim 33, wherein one ormore thin-film storage transistors of the reference memory string areprogrammed to have set reference threshold voltages.
 36. The memoryarray of claim 1, wherein one or more pf tje NOR-type memory strings aredesignated spare strings, each spare string being configurable toreplace a NOR-type memory string in the memory array.
 37. The memorystring of claim 33, wherein threshold voltages programmed into thin-filmstorage transistors of the reference memory string correspond toprogrammed states under a multi-bit scheme.
 38. The memory array ofclaim 1, wherein the channel region in each thin-film storage transistoris formed subsequent in time to forming the common drain region and thecommon source region.
 39. The memory array of claim 38, wherein thechannel region of each thin-film storage transistor has a lengthsubstantially determined by a thickness of a sacrificial layer removed.40. The memory array of claim 1, wherein the circuitry in thesemiconductor substrate comprise a data integrity circuit.
 41. Thememory array of claim 40, wherein the data integrity circuit comprisesone or more of: a data parity circuit and an error correcting circuit.